ThreadGroup in Java

Java provides a convenient way to group multiple threads in a single object. In such way, we can suspend, resume or interrupt group of threads by a single method call.

Note

Now suspend(), resume() and stop() methods are deprecated.

Java thread group is implemented by java.lang.ThreadGroup class.

A ThreadGroup represents a set of threads. A thread group can also include the other thread group. The thread group creates a tree in which every thread group except the initial thread group has a parent.

A thread is allowed to access information about its own thread group, but it cannot access the information about its thread group's parent thread group or any other thread groups.

Constructors of ThreadGroup class

There are only two constructors of ThreadGroup class.

No.	Constructor	Description
1)	ThreadGroup(String name)	creates a thread group with given name.
2)	ThreadGroup(ThreadGroup parent, String name)	creates a thread group with given parent group and name.

Methods of ThreadGroup class

There are many methods in ThreadGroup class. A list of ThreadGroup methods are given below.

S.N.	Modifier and Type	Method	Description
1)	void	checkAccess()	This method determines if the currently running thread has permission to modify the thread group.
2)	int	activeCount()	This method returns an estimate of the number of active threads in the thread group and its subgroups.
3)	int	activeGroupCount()	This method returns an estimate of the number of active groups in the thread group and its subgroups.
4)	void	destroy()	This method destroys the thread group and all of its subgroups.
5)	int	enumerate(Thread[] list)	This method copies into the specified array every active thread in the thread group and its subgroups.
6)	int	getMaxPriority()	This method returns the maximum priority of the thread group.
7)	String	getName()	This method returns the name of the thread group.
8)	ThreadGroup	getParent()	This method returns the parent of the thread group.
9)	void	interrupt()	This method interrupts all threads in the thread group.
10)	boolean	isDaemon()	This method tests if the thread group is a daemon thread group.
11)	void	setDaemon(boolean daemon)	This method changes the daemon status of the thread group.
12)	boolean	isDestroyed()	This method tests if this thread group has been destroyed.
13)	void	list()	This method prints information about the thread group to the standard output.
14)	boolean	parentOf(ThreadGroup g	This method tests if the thread group is either the thread group argument or one of its ancestor thread groups.
15)	void	suspend()	This method is used to suspend all threads in the thread group.
16)	void	resume()	This method is used to resume all threads in the thread group which was suspended using suspend() method.
17)	void	setMaxPriority(int pri)	This method sets the maximum priority of the group.
18)	void	stop()	This method is used to stop all threads in the thread group.
19)	String	toString()	This method returns a string representation of the Thread group.

Let's see a code to group multiple threads.

Example

snippet

ThreadGroup tg1 = new ThreadGroup("Group A"); 
Thread t1 = new Thread(tg1,new MyRunnable(),"one");   
Thread t2 = new Thread(tg1,new MyRunnable(),"two");   
Thread t3 = new Thread(tg1,new MyRunnable(),"three");

Now all 3 threads belong to one group. Here, tg1 is the thread group name, MyRunnable is the class that implements Runnable interface and "one", "two" and "three" are the thread names.

Now we can interrupt all threads by a single line of code only.

Thread.currentThread().getThreadGroup().interrupt();

Example

ThreadGroup Example

File: ThreadGroupDemo.java

snippet

public class ThreadGroupDemo implements Runnable{
	public void run() {
	      System.out.println(Thread.currentThread().getName());
	}
   public static void main(String[] args) {
      ThreadGroupDemo runnable = new ThreadGroupDemo();
          ThreadGroup tg1 = new ThreadGroup("Parent ThreadGroup");
          
          Thread t1 = new Thread(tg1, runnable,"one");
          t1.start();
          Thread t2 = new Thread(tg1, runnable,"two");
          t2.start();
          Thread t3 = new Thread(tg1, runnable,"three");
          t3.start();
             
          System.out.println("Thread Group Name: "+tg1.getName());
	     tg1.list();

    }
   }

Output

one two three Thread Group Name: Parent ThreadGroup java.lang.ThreadGroup[name=Parent ThreadGroup,maxpri=10] Thread[one,5,Parent ThreadGroup] Thread[two,5,Parent ThreadGroup] Thread[three,5,Parent ThreadGroup]